feat(frontend): English UI + 'Datamart Builder with Human In The Loop' tagline

The interface is now fully English regardless of the chat language (session
titles stay in whatever language the user typed). Changed the right-rail
subtitle to 'Datamart Builder with Human In The Loop'. Translated all chrome:
new-question/settings dialogs, workflow phase strip (F1-F8), session rail,
steer input, widgets (select/multiselect/freetext/fallback/reserved controls)
and viewers (SQL, results, schema linking). Test matchers updated in lockstep.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/shell/AppShell.tsx

@@ -61,7 +61,7 @@ export function AppShell() {
             Thoth<span className="text-primary">II</span>
           </h1>
           <p className="mt-1.5 text-[0.7rem] uppercase tracking-[0.14em] text-muted-foreground">
-            Query in linguaggio naturale
+            Datamart Builder with Human In The Loop
           </p>
         </div>
 
@@ -72,7 +72,7 @@ export function AppShell() {
 
         <div className="px-4 pb-1.5 pt-1">
           <span className="text-[0.7rem] font-bold uppercase tracking-[0.12em] text-primary">
-            Sessioni
+            Sessions
           </span>
         </div>
         <div className="flex-1 overflow-y-auto px-2 pb-4">
@@ -95,8 +95,9 @@ function EmptyState() {
         Thoth<span className="text-primary">II</span>
       </span>
       <p className="mt-4 max-w-md text-balance text-[0.95rem] leading-relaxed text-muted-foreground">
-        Interroga il tuo data warehouse in linguaggio naturale. Crea una nuova
-        domanda dal pannello a destra per avviare una sessione guidata.
+        Build datamarts from your data warehouse through a guided,
+        human-in-the-loop workflow. Start a new question from the panel on the
+        right to begin a session.
       </p>
     </div>
   );